Continuous Resistivity Profiling in Shallow Marine and Freshwater Environments
D.D. Snyder, Scott C. MacInnes, M.J. Raymond, and Kenneth L. Zonge, Zonge Engineering and Research Organization, Tucson, Arizona, U.S.A.
Paper – [pdf] IP_MarineProfiling
In this paper, we describe an instrument system for performing continuous resistivity profiling in shallow freshwater and marine environments. Using a streamer cable containing 9 electrodes, the system continuously samples the dipole-dipole resistivity at n-spacings 1 through 6. The system can be installed aboard a variety of small inboard or outboard powered vessels in a few hours. Hand-held or marine GPS units provide location information that is recorded by a laptop computer. With this system, up to 40 line-km/day of dipole-dipole data have been collected. The resistivity data are merged with the GPS positions as a post-processing step.
The final step in the post-processing is the inversion of overlapping segments of each profile using a 2-D smooth model. The inversions provide high resolution images of the geo-electric cross-section. The depth of investigation ranges from 20-30 m, with a 10 m dipole spacing.
Over the last four years, we have performed surveys on the Ohio River, near Louisville, KY, on tidal estuaries and bays along the Atlantic coast in Delaware, Maryland, Virginia, and North Carolina, and in Tampa Bay, Florida. Data from these surveys will be used to illustrate the final deliverable from a survey.
The electrical resistivity or conductivity of the shallow sub-surface is of particular interest to environmental geophysicists. It is a physical property directly affected not only by the presence of conducting foreign material that may have been improperly disposed of, but also by the chemistry of the saturating fluids. Environmental hydrologists are sometimes able to use resistivity measurements to map contaminant plumes as they migrate down the hydraulic gradient from
their source. Moreover, hydrologists know that fluid migration within our aquifers is an important source of contamination in our lakes, rivers, and tidal estuaries. Thus, resistivity measurements can be valuable for investigation of shallow ground-water hydrology from the water side as well as the land side. Our purpose here is to describe [the Zonge] system for measuring resistivity and IP in shallow water, and the data processing involved.